In general, spiral-type flexible pipes are pipes designed to have a smaller fluid resistance than ring-type flexible pipes, to facilitate heat exchange between a fluid and the outside, and to be freely bent. In order to connect a spiral-type flexible pipe to another pipe, a heat storage tank, a boiler, etc., a connector using a screw-connection method, as an alternative to a welding method, is used. However, such a connector causes damage to parts, such as a packing, due to a difference of thermal expansion/contraction coefficients, thereby causing water leakage.
Further, a metal joining method (flat or joint method) without a packing (sealer) is used. However, this method requires an operation to flare a connection part at the tip of a pipe, which cannot be carried out in the field, and requires large tightening torque, thereby lowering operation efficiency.